1. Field
The present invention relates generally to data communication, and more specifically to techniques for determining transport format combinations (TFCs) supported for use in normal and compressed modes in a wireless (e.g., W-CDMA) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication including voice and packet data services. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of system, including increased system capacity. A CDMA system is typically designed to conform to one or more standards, such as IS-95, cdma2000, and W-CDMA standards, all of which are known in the art and incorporated herein by reference.
The W-CDMA standard supports data transmission on one or more transport channels, and each transport channel may be associated with one or more transport formats (TFs) that may be used for the data transmission. Each transport format defines various processing parameters such as the transmission time interval (TTI) over which the transport format applies, the size of each transport block of data, the number of transport blocks within each TTI, the coding scheme to be used for the transport blocks in a given TTI, and so on. The use of multiple transport formats for a given transport channel allows different types or rates of data to be transmitted over the same transport channel. At any given moment, a specific transport format combination (TFC), which comprises one transport format for each transport channel, is selected from among a number of possible transport format combinations and used for all transport channels.
The W-CDMA standard also supports a “compressed mode” of operation on the uplink whereby data is transmitted from a terminal to a base station within a shortened time duration (i.e., compressed in time). The compressed mode is used in W-CDMA to allow a terminal in active communication with the system (i.e., on a traffic channel) to temporarily leave the system in order to perform measurements on a different frequency and/or a different Radio Access Technology (RAT) without losing data from the system. In the compressed mode for the uplink, data is transmitted by the terminal during only a portion of a (10 msec) frame so that the remaining portion of the frame (referred to as a transmission gap) may be used by the terminal to perform the measurements.
In accordance with the W-CDMA standard, the reduction in the transmission time for a compressed frame may be achieved by (1) reducing the amount of data to transmit in the frame, (2) increasing the coding rate, or (3) increasing the data rate. Reducing the amount of data to transmit in the compressed frame may be impractical for some applications, such as voice, since the data reduction may result in significantly reduced quality of service. Increasing the coding rate or data rate may be possible if the transmit power for the compressed frame is increased such that the energy-per-bit-to-total-noise-plus-interference ratio (Eb/Nt) for the compressed frame is similar to that for a non-compressed frame.
As noted above, a number of transport channels may be concurrently supported and a set of transport formats may be defined for each transport channel. A set of “configured” transport format combinations may be defined for the transport channels, with each such transport format combination being associated with a particular relative transmit power level needed to achieve a target block error rate (BLER). The required transmit power for each transport format combination is dependent on (1) whether or not the terminal is in the compressed mode and (2) the parameter values defining the compressed transmissions in the compressed mode. To achieve high system performance, only the configured transport format combinations supported by the terminal's maximum transmit power at the current channel conditions (i.e., those that can be transmitted with the required power for achieving the target block error rate) should be identified as those that may be selected for use. And only one specific transport format combination would then be selected from this set of supported transport format combinations for actual use at the next frame (shortest TTI) boundary.
There is therefore a need in the art for techniques for determining transport format combinations supported for use in normal and compressed modes in a W-CDMA system.